1. Field of the Invention
The present invention generally relates to an optical semiconductor module and a light receiving element. More particularly, the present invention relates to an optical semiconductor module that includes a light emitting element and a light receiving element, and to the light receiving element.
2. Description of the Related Art
An optical semiconductor module having a light emitting element such as a semiconductor laser mounted thereon is normally used for optical communications and the likes. For example, in an optical semiconductor module to be used for optical communications, the light emitted from a light emitting element is expected to have constant intensity. Therefore, such an optical semiconductor module includes a light emitting element and a light receiving element, and the light receiving element receives part of the light emitted from the light emitting element. The output of the light receiving element is fed back to the light emitting element, so as to maintain constant light emission of the light emitting element.
FIG. 1 is a schematic view of the above described conventional optical semiconductor module. A light emitting element 62 that is a laser diode (LD) chip is mounted on a module package 50 via a special-purpose mounting carrier 53. A light receiving element 80 that is a planar-type photodiode (PD) chip is mounted on the package 50 via a mounting carrier 51. An optical fiber 56 and a collecting lens 54 are fixed onto the package 50 via a fixing unit (not shown). The light emitting element 62 has an activation layer 66 that emits light. The light receiving element 80 has a light absorption layer 84 and a light transmission layer 85, and an impurity diffusion area 86 that is a light receiving area is formed in the light transmission layer 85.
Forward emitted light 70 that is emitted from the front side face 67 of the light emitting element 62 enters the optical fiber 56 through the collecting lens 54. Meanwhile, backward emitted light 72 is emitted from the rear side face 68, and enters the impurity diffusion area 86 of the light receiving element 80. The backward emitted light 72 is the light to be used for monitoring the light emission intensity of the forward emitted light 70. Therefore, the intensity of the backward emitted light 72 may be much lower than the intensity of the forward emitted light 70. The light receiving element 80 (a LD monitor) outputs an electric signal in accordance with the light intensity of the emitted light 72. Based on the electric signal, a control unit (not shown) controls the light emission intensity of the light emitting element 62. In this manner, the light emission intensity of the forward emitted light 70 can be maintained at a constant level.
Japanese Unexamined Patent Publication Nos. 5-175614 and 10-321900 disclose optical semiconductor modules. In each of the optical semiconductor modules, the mounting face of the light emitting element is parallel to the mounting face of the light emitting element, and the light emitted from the light emitting element is reflected and then enters the light receiving element. Japanese Unexamined Patent Publication No. 59-96789 discloses an optical semiconductor module in which the mounting face of the light emitting element is parallel to the mounting face of the light receiving element, and the light receiving element is mounted directly on the mounting face, which is different from the conventional structure shown in FIG. 1.
A light emitting element such as a LD emits light from a face that is a side face with respect to the mounting face (the face to be mounted on a mounting unit or the like). Meanwhile, a light receiving element such as a PD receives light through a face that is the upper face with respect to the mounting face. Therefore, in the conventional structure shown in FIG. 1, the light emitted from the rear side face 68 of the light emitting element 62 is received in the impurity diffusion area on the upper face of the light receiving element 80. To realize such a structure, the mounting face of the light receiving element 80 should be made perpendicular to the mounting face of the light emitting element 62. Therefore, the mounting carrier 53 of the light emitting element 62 needs to be prepared separately from the mounting carrier 51 of the light receiving element 80, as shown in FIG. 1. As a result, the number of assembling procedures, the number of required components, and the production costs increase accordingly.
In each of the structures disclosed in Japanese Unexamined Patent Publication Nos. 5-175614 and 10-321900, the light emitting element and the light receiving element can be mounted on the same mounting carrier. However, it is necessary to prepare a component that reflects the light emitted from the light emitting element. As a result, the production costs become higher. By the technique disclosed in Japanese Unexamined Patent Publication No. 59-96789, there is no need to prepare such a component, and the increase in production costs is not as large as that in the case of the conventional structure shown in FIG. 1. However, since the planar-type light receiving element is mounted parallel to the light emitting element, the photosensitivity becomes lower than that in the case of the conventional structure shown in FIG. 1. Furthermore, since extra light from the light emitting element enters through the light absorption layer outside the light receiving face, there is the problem that most of the incident light is not converted into electric signals.